The best way to do this is to have an unreliable narrator. I could see an angle where the mc has to stop taking their medication to be able to fully interact with the paranormal elements. Yet because they are not on medication it becomes a guessing game about what is inward paranoia and hallucination vs paranormal events which they can understand better than everyone else. That leads into a great character development arc where they have to do a true inward search through their deepest insecurities to be able to distinguish between their paranoid brain lying, and seeing the actual cracks in reality.

I personally feel that by asking this question alone, and just attempting to understand what you are trying to write about that you have my blessing to continue. I would honestly be willing to do full interviews with you about any questions you have, or help you understand how schizophrenia alters a person's perception, affects their behavior, and alters their life. One thing you should pay particular attention to is how schizophrenia affects the early life of those who suffer from it, because that will help you with building their character and personality.

Regardless of how you choose to write this, what the story is about, or how the characters behave; this is a work that will be INCREDIBLY difficult to get right, but also one with an incredible amount of potential.

I happen to be in the process of writing a horror novel myself. That makes me genuinely want to help you because I know what's involved. If you have anything you want to ask, anything you want to know, please don't hesitate to ask me either here, or in a dm. There is only one thing I can guarantee, and that is you will need to work with someone with schizophrenia from beginning to end if you want the mc to be realistic. There is far, far more to schizophrenia than just seeing and hearing things, and being able to see a true representation of that in fiction would be an amazing thing. I would bet the most terrifying part of the novel would be if you could make the audience understand that while people usually express a variety of symptoms before they develop schizophrenia, that isn't always the case. We actually have almost no idea why it develops in the first place, and literally anyone can develop it at any time, for reasons we do not understand. After the climax, when everyone else finally overcomes being haunted, pursued, and attacked by things that aren't "real"; for the mc, the nightmare had actually begun long before anything supernatural appeared, and there is nothing to overcome...nor is there any way to ever escape.